Heating means for bimetal trip element



April 1966 A. STROBEL I 3,244,837

HEATING MEANS FOR BIMETAL TRIP ELEMENT Filed Dec. 17, 1962 United States Patent 3,244,837 HEATlNG MEANS FOR BIMETAL TRIP ELEMENT Albert Strobel, Cherry Hill, N.J., assignor to I-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Dec. 17, 1962, Ser. No. 244,989

Claims. (Cl. 200116) This invention relates to circuit breakers in general and more particularly to a construction for an automatic trip unit having a novel bimetalheater arrangement.

In molded case circuit breakers, especially in the higher current rated units, the current carrying and contact operating elements are closely spaced within the housing. Trip units for these breakers are usually provided with bimetals which are indirectly heated by means of laminated conductors. The laminated conductors are arranged to also supply current for energization of the instantaneous magnetic trip structure. v

A laminated heater is required since it is necessary to provide for flexibility between the bimetal and the heater terminals. The bimetal must be mounted rigidly to maintain calibration settings for time delayed tripping while. the conductor between the heater terminals and the bimetal must be flexible to allow for distortion which often occurs when interchangeable trip units are mounted in different base assemblies.

Industry is constantly striving to reduce the size of circuit interrupters for a given rating and as a result is striving to reduce the size of trip units for these interrupters. A major obstacle to be overcome in reducing the size of such a trip unit is the substantialthickness of the laminated heater required to carry the entire phase current without overheating the entire interrupter. In an attempt to overcome this obstaclethe prior art has utilized solid heater constructions, but in so doing has sacrificed calibration accuracy.

In the device of the instant invention the conventional laminated heater is combined with a solid heater with these heaters being connectedin electrical parallel. The solid portion is provided with a reduced cross-section 'between the points where the laminated portion is fastened. The laminated portion and the solid portion of reduced cross-section are so proportioned that a large percent of the total phase current flows through the solid portion creating some heat whereas a relatively smaller portion flows through the laminated portion. The laminated portion is constructed of relatively few layers with -a relatively high total resistance and as a result furnishes most of the heat to operate the bimetal. In addition, the current through the laminated portion is suflicient to operate the magnetic structure. It has been found that the solid heater portion may be located in a section of the trip unit where space requirements are not as critical as in most other sections of the trip unit. Further, by reducing the number of laminations required the width of the trip unit may be reduced considerably.

Accordingly, a primary object of this invention is to provide a circuit interrupter having a novel trip unit construction.

Another object is to provide a trip unit having a novel bimetal heater construction.

Still another object is to provide a bimetal heater having a solid portion and a laminated portion in shunt with the solid portion with the laminated portion arranged to carry current for energization of a magnetic trip unit.

3,244,837 Patented Apr. 5, 1966 ice A further object is to provide a bimetal heater having parallel solid and laminated portions with the impedances of these portions being proportioned such that most of the heat for bimetal deflection results from the passage of current through the laminated portion.

These as well as other objects of this invention shall become readily apparent after reading the following description of the accompanying drawings in which:

FIGURE 1 is a side elevation showing current carrying and operating elements in the center phase of a three phase circuit breaker which includes a trip unit having a bimetal heater constructed in accordance with the teachings of the instant invention.

FIGURE 2 is an end view of the trip unit looking in the direction of arrows 22 of FIGURE 1.

FIGURE 3 is a perspective illustrating the relationship between the bimetal, the laminated heater portion and the solid heater portion.

Now referring to the figures. Circuit interrupter 10 is a three phase molded case unit having a contact operating mechanism 11 of a type described in detail in United States Patent 2,932,706, issued April 12, 1960, to A. Bo-denschatz, entitled Hold Open and Anti-Rebound Latches. Since contact operating mechanism 11 forms no part of this invention, mechanism 11 shall not be desicribedherein except in relation to the trip unit.

Each phase of circuit breaker 10 is provided with a current path extending between line terminal 12 and load terminal 13 as follows: line terminal 12 through stationary contact 14 mounted thereon, to movable contact 15 and contact arm 16 carrying contact 15 at one end thereof, through flexible braid 17 mounted to the other end of contact arm 16, to clip 18 connected to the left end of load terminal 13 by fastening means 19,

A portion 20 of load terminal 13 intermediate its ends is ofreduced cross-section and is shunted by looped laminated member 21. member 21 at the ends thereof to load terminal 13. Rivets 24 secure the lower end of'bimetal 25 to leg 21a of laminated member 21' and to plate 26. Studs 27 extend through magnetic core 28, frame 29, spacers 30, plate 26, insulating spacer 31 and insulating wall 32 from load side to line side thereof. Disposed 0n the line side of the wall 32 arenuts'33 in threaded engagement with screws 27. As is Well-known to the art, wall 32 and the elements secured thereto constitute a three phase interchangeable trip unit. U

The upper'end of bimetal 25 is free to deflect to the right with respect to FIGURE 1 when heated. Upon be: ing deflected to the right the free end of bimetal 25 engages adjusting screw 35 extending through extension 36 of common trip bar 37 causing trip bar 37 to pivot in a clockwise direction with respect to FIGURE 1. As trip bar 37 pivots clockwise latching surface 38 carried thereby moves away from tip 39 of member 40. Latch 40 is pivotally mounted at a point intermediate its ends to shaft 41 which is maintained in a horizontal position by brackets 42, 43 secured to wall 32 on the line side thereof. Latch surface 44 at the other end of member 40 supports one end of intermediate latch member 45 whose other end provides a surface 46 for engaging the latching tip 41 of operating mechanism cradled 48.

Operating mechanism spring 49 biases latch tip 47 upward and in so doing exerts a force on intermediate latch member 45 urging it clockwise about its pivot 51. This in turn urges latch member 40 in a counterclockwise direction about its pivot 41. Thus, when latch surface 38 releases latch tip 39 latch member 40 is permitted to rotate counterclockwise and intermediate latch member Fasteners 22, 23 secure laminated 3 45 is permitted to rotate clockwise thereby releasing cradle 48 and tripping contact operating mechanism 11.

Magnet 28 is a U-shaped member with leg 21b of laminated member 21 passing between the arms of magnet 28 to provide the energizing turn therefor. Sheet mica 99 is disposed between laminated member 21 and plate 26 as well as between member 21 and magnet 28. Frame 29 is provided with apertures 51 which receive downward extensions of armature 52 and constitute a pivot point for armature 52. Tension spring 53, connected at its lower end to frame 29 and at its upper end to armature 52, biases armature 52 to the position shown in FIG- URE 1.

When the magnitude of current passing through laminated member 21 exceeds a predetermined level, sufficient flux is generated by magnet 23 to attract armature 52. This causes adjusting screw 54, mounted at the upper end of armature 52, to move to the left with respect to FIGURE 1 into engagement with downward extension 55 of common t-rip bar 37. The engagement between screw 54 and extension 55 brings about a clockwise rotation of common trip bar 37 causing circuit breaker to trip open in the manner previously explained.

The cross-sectional areas of solid necked-down portion and laminated member 21 are so proportioned that sutficient heat is generated for controlled deflection of bimetal 25. Typically, the impedances of laminated member 21 and necked-down portion 20 are such that a large percentage of the total current flows through neckeddown portion 20 creating some heat whereas a relatively small portion of the current flows through laminated portion 21. Laminated portion 21 is constructed of relatively few laminations so as to have a relatively high total resistance with the result being that the current flowing through laminated member 21 generates most of the heat causing deflection of bimetal 25. It is noted that the current magnitude through laminated member 21 upon the occurrence of an overload condition is sufficient to operate the magnetic trip means.

Thus, this invention provides a trip unit in which the bimetal trip element is heated substantially entirely by indirect means. That is, the bimetal is arranged externally of the current path through the breaker. The bimetal heater includes a laminated heater shunted by a solid necked-down portion of the line terminal member. This construction results in a compact unit which maintains calibration accuracy.

Although there has been described a preferred embodiment of this novel invention, many variations and modifications will now be apparent to those skilled in the art. Therefore, this invention is to be limited, not by the specific disclosure herein, but only by the appending claims.

The embodiments of the invention in which an exclusive privilege or property is claimed are defined as follows:

1. A multiphase circuit breaker including cooperating contact means for each phase; a mechanism connected to said contact means for operating the same into and out of engagement; a heat responsive non-current carrying automatic trip element for each phase arranged to cause said mechanism to operate said cooperating contact means of all phases out of engagement upon the occurrence of predetermined circuit conditions; a substantially flat conducting strip for each of said. phases connected in electrical series with the cooperating contact means thereof; a substantial-1y U-shaped heater individual to each of said trip elements with the opposite ends of said heaters being electrically connected to its associated conductive strip to shunt a portion of the conducting strip of its respective phase; said portion of the conducting strip having a cross sectional area substantially less than the remaining cross-sectional area of said conducting strip; said heater being formed to provide a high resistance both for adequate heating of the bimetal and minimum shunting of current from said conducting strip.

2. The device of claim 1 wherein said heaters are mounted directly upon the conducting strips associated therewith.

3. The device of claim 1 wherein each of said portions of the conducting strips and the heaters associated therewith have their electrical resistances proportioned so that more current flows through said portions of said strips than flows through said heaters.

4. The device of claim 1 wherein each of said U- shaped heaters are comprised of a plurality of conductive laminations.

5. The device of claim 4 wherein the Ushaped member is comprised of a plurality of conductive laminations with the number of laminations being selected to provide a relatively high total resistance so that the current flowing therethrough generates most of the heat causing deflection of the heat responsive non-current carrying automatic trip element.

References Cited by the Examiner UNITED STATES PATENTS 2,047,739 7/ 1936 Lingal 200-122 2,050,284 8/ 1936 Dorfman 200-122 2,074,584 3/1937 Graves 200122 2,439,069 4/ 1948 Anderson 200-116 2,629,796 2/ 1953 Kern 2001 16 2,795,670 6/ 1957 Cellerini et al 200 -88 3,097,277 7/1963 Wingard 20088 3,139,497 6/ 1964 Geissner 200-416 BERNARD A. GILHEANY, Primary Examiner. 

1. A MULTIPHASE CIRCUIT BREAKER INCLUDING COOPERATING CONTACT MEANS FOR EACH PHASE; A MECHANISM CONNECTED TO SAID CONTACT MEANS FOR OPERATING THE SAME INTO AND OUT OF ENGAGEMENT; A HEAT RESPONSIVE NON-CURRENT CARRYING AUTOMATIC TRIP ELEMENT FOR EACH PHASE ARRANGED TO CAUSE SAID MECHANISM TO OPERATE SAID COOPERATING CONTACT MEANS OF ALL PHASES OUT OF ENGAGEMENT UPON THE OCCURRENCE OF PREDETERMINED CIRCUIT CONDITIONS; A SUBSTANTIALLY FLAT CONDUCTING STRIP FOR EACH OF SAID PHASES CONNECTED IN ELECTRICAL SERIES WITH THE COOPERATING CONTACT MEANS THEREOF; A SUBSTANTIALLY U-SHAPED HEATER INDIVIDUAL TO EACH OF SAID TRIP ELEMENTS WITH THE OPPOSITE ENDS OF SAID HEATERS BEING ELECTRICALLY CONNECTED TO ITS ASSOCIATED CONDUCTIVE STRIP TO SHUNT A PORTION OF THE CONDUCTING STRIP OF ITS RESPECTIVE PHASE; SAID PORTION OF THE CONDUCTING STRIP HAVING A CROSS SECTIONAL AREA SUBSTANTIALLY LESS THAN THE REMAINING CROSS-SECTIONAL AREA OF SAID CONDUCTING STRIP; SAID HEATER BEING FORMED TO PROVIDE A HIGH RESISTANCE BOTH FOR ADEQUATE HEATING OF THE BIMETAL AND MINIMUM SHUNTING OF CURRENT FROM SAID CONDUCTING STRIP. 